Adding to the function docs that groupby.transform() function parameter can be a string

[seanjedi]

Docs change: Adding to the function docs that groupby.transform() function parameter can be a string

• closes DOC: Docs for groupby.transform() should mention that function parameter can be a string in transform  #49961
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[WillAyd]

Thanks @seanjedi

[ramvikrams]

@WillAyd I feel examples should be added for both, DataFrameGroupBy.transform() and SeriesGroupBy.transform() with strings